X-ray ct apparatus

ABSTRACT

There is provided an X-ray CT apparatus including: an X-ray source; a wedge which is disposed between the X-ray source and a subject and in which a shield blocking a part of an X ray is formed; a wedge driving unit for moving position of the wedge; and a system control unit controlling the wedge driving unit during a scan execution period to control the position of the wedge.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/JP2011/006890, filed on Dec. 9, 2011, which is based upon and claimsthe benefit of priority from the prior Japanese Patent application No.2011-001947, filed on Jan. 7, 2011, the entire contents of which areincorporated herein by reference.

FIELD

An embodiment of the present invention relates to an X-ray CT apparatus.

BACKGROUND

In a gantry of an existing X-ray CT (Computed Tomography) apparatus, acollimator is provided for a front face of an X-ray tube field. Thecollimator has a wedge for adjusting cutting of a soft ray and theintensity distribution of an X ray, and a slit mechanism whichopens/closes in accordance with slice thickness at the time of a scan.The collimator forms a fan beam whose radiation dose is optimized, andirradiates a subject with an X ray.

In the case where the slit mechanism has a biaxial opening/closingmechanism in which right and left slits operate singularly, to suppressunnecessary irradiation of X rays generated before and after a scan,active collimation operation of controlling the width and position of anopening by varying the timings of opening the right and left slits tooptimize the radiation dose can be performed. To execute the activecollimation to reduce the dose of radiation to the subject, generally, aslit mechanism in an optical system has to have a biaxialopening/closing mechanism which can open/close two slits singularly.However, since the system is much more expensive than a normal opticalsystem having no biaxial opening/closing mechanism in which two slitsopen/close, it is demanded to realize the active collimation operationby an inexpensive slit mechanism in which slits are opened/closedsimultaneously.

According to one embodiment, an inexpensive high-performance X-ray CTapparatus with reduced radiation dose is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block configuration diagram of an X-ray CT apparatus in afirst embodiment.

FIGS. 2A and 2B are configuration diagrams of a wedge in the X-ray CTapparatus in the embodiment.

FIG. 3 is an explanation diagram of active collimation.

FIG. 4 is a flowchart diagram of active collimation operation in theembodiment.

FIGS. 5A to 5C are explanatory diagrams of the active collimationoperation by a wedge in the embodiment.

FIGS. 6A and 6B are configuration diagrams of a wedge in an X-ray CTapparatus in a second embodiment.

FIG. 7 is a block configuration diagram of a wedge driving unit in athird embodiment.

FIG. 8 is a time chart diagram of active collimation operation in theembodiment.

DETAILED DESCRIPTION

According to one embodiment, an X-ray CT apparatus of an embodimentincludes: an X-ray source; a wedge which is disposed between the X-raysource and a subject and in which a shield blocking a part of an X rayis formed; a wedge driving unit for moving position of the wedge; and asystem control unit controlling the wedge driving unit during a scanexecution period to control the position of the wedge.

Hereinafter, embodiments will be described in detail below withreference to the drawings of FIGS. 1 to 8. There are various types ofX-ray CT apparatuses such as a type in which an X-ray tube and an X-raydetector are disposed opposite to each other and revolve together arounda subject and a type in which a number of detection elements are arrayedin a ring shape and only an X-ray source revolves about a subject.Embodiments are applicable to any of the types. In the embodiments, atype in which an X-ray tube and an X-ray detector revolve together willbe explained.

First Embodiment

FIG. 1 shows an X-ray CT apparatus in a first embodiment. The X-ray CTapparatus of the embodiment has a gantry 11 for scanning a subject P(patient) with an X ray, a bed 12 by which the subject P is moved intothe gantry 11, a system control unit 13 for controlling the entire X-rayCT apparatus, and a reconstruction unit 14 as a computer for processingprojection data obtained from the gantry 11 and reconstructing an imagefor medical use.

The bed 11 is constructed by a revolving part 15 which revolves aroundthe subject P (patient) as a center and a stationary part 16 as theother part. The revolving part 15 has an X-ray tube 111 which generatesan X ray, a wedge 112 for adjusting a distribution of dose of the X raygenerated from the X-ray tube 111 to a fan angle direction, an X-raydetector 113 for detecting the X ray which passed through the subject P(patient), a data acquisition system (DAS) 114 for converting the datadetected by the X-ray detector 113 to digital data and acquiring thedigital data, and a contactless data transmission system 115 forcontactlessly transmitting the projection data acquired by the dataacquisition system 114 to the reconstruction unit 14 on the outside ofthe revolving part 15.

The system control unit 13 has a high-voltage generating apparatus 116for generating voltage to be applied to the X-ray tube 111 forgenerating an X ray, a wedge driving unit 117 for moving the position ofthe wedge in accordance with a scanning parameter, a revolution drivingunit 118 for making the revolving part 15 revolve on the basis of ascanning parameter of a helical scan or the like, and a bed driving unit119 for moving the bed 12 on which the subject P lies into the gantry11.

The reconstruction unit 14 reconstructs an image for medical use whichis necessary for diagnosis from the projection data transferred from thecontactless data transmission system 115. The reconstructed image formedical use is displayed on a not-shown monitor or the like connected tothe reconstruction unit 14. The system control unit 13 and thereconstruction unit 14 are constructed generally based on a computerhaving high processing capability.

FIGS. 2A and 2B are configuration diagrams of the wedge in the X-ray CTapparatus in the embodiment. FIG. 2A is a perspective view, and FIG. 2Bis a cross sectional view taken along the Z-Z′ axis shown by a dottedline, and seen along line X-X′. XB indicates spread of an X-ray beam.

The wedge adjusts the X-ray dose distribution in the fan angle directionand has a U-shaped opening in its center. A plurality of kinds of wedgeshaving different opening shapes are used in accordance with the case ofscanning the subject P from the front, the case of scanning the subjectP from the side, and the like, and are selected according to the scanparameters.

Since the energy of a soft X-ray is low, in reality, even when a softX-ray is emitted from the X-ray tube 111, it is often absorbed by thesubject P and does not reach the detector 113. The wedge 112 thereforealso has the effect of cutting soft rays. Usually, metal such asaluminum is used for the wedge 112.

As shown in FIG. 2A, the wedge of the embodiment is obtained by addingan X-ray shield 22 to an upper part (on the X-ray tube 111 side) of anormal wedge 21. FIG. 2B shows the X-ray beam XB generated from an X-rayfocal point 23 and, in the case where the X-ray focal point 23 existsaround the center of the wedge 21, the X-ray shield 22 has to have, forexample, a slit shape as shown in FIG. 2A by which the X ray is notblocked. In the case where the X-ray focal point 23 is deviated from thecenter of the wedge 21, as shown in FIG. 2B, the X-ray shield 22 has ashape like a canopy for the side face of the wedge 21. The thickness ofthe X-ray shield 22 is set so that most of the X ray is absorbed and isnot passed. Usually, a metal which blocks an X ray such as lead can beused for the X-ray shield 22.

Referring now to FIG. 3, active collimation will be described. Thehorizontal axis of FIG. 3 indicates time of executing one scan, and thevertical axis indicates width of irradiation in the slice thicknessdirection (the body axis direction of the subject P). That is, thediagram shows how the width of irradiation to the subject P changesduring execution of one scan. The irradiation width is almostproportional to the opening width of the not-shown slit mechanism.

To perform a helical scan, until an X ray is actually emitted to start ascan, acceleration time for making the speed of the revolving part 15 ofthe gantry constant, time for deceleration since the end of the scanuntil stop, running time until the moving speed of the bed 12 becomesconstant, and the like are necessary. However, the times are notincluded in FIG. 3. “Start of scan” in FIG. 3 indicates the head ofirradiation of an X ray and acquisition of data necessary forreconstruction, and “end of scan” indicates end of the data acquisitionand end of irradiation of the X ray.

To reconstruct an image for medical use by a helical scan, an X ray hasto be applied wider than an actual reconstruction region. Therefore,before and after the scan execution time, Excess X-ray irradiationranges are necessary in addition to a reconstruction range. Theadditional X-ray irradiation ranges are defined as “non-reconstructionranges” as ranges unnecessary for reconstruction.

In the non-reconstruction ranges before and after the scan, if the slitopening width is the same as that in the reconstruction range, due tothe spread of the X-ray beam XB, a region part unnecessary forreconstruction in the subject P is also irradiated, and radiation doseis not optimized. Consequently, in a slit mechanism in which two slitscan open/close singularly, in the non-reconstruction range at the startof a scan, by moving one of the slits to reduce the slit opening widthwhich is determined according to the slice thickness to half, the widthof irradiation to the subject P is reduced to half. At a time pointafter the non-reconstruction range, the slit opening width determinedaccording to the slice thickness is set. In the non-reconstruction rangeat the end of the scan, the slit opposite to the scan start position isdriven to reduce the width of irradiation to the subject P to half. Bysuch active collimation operation, the excess X-ray irradiation beforeand after the scan, is suppressed and the X-ray radiation dose isoptimized.

In the embodiment, by using the wedge shown in FIGS. 2A and 2B, theactive collimation operation for reducing X-ray radiation dose can beperformed also by the slit mechanism having no biaxial opening/closingmechanism in which two slits can open/close singularly. Therefore, it isassumed that an X ray is applied to the subject P with the same slitopening width during scan execution.

FIG. 4 is a flowchart diagram of active collimation operation in theembodiment. FIGS. 5A to 5C are diagrams for explaining the activecollimation operation using the wedge in the embodiment. FIG. 5A is anexplanatory diagram at the start of a scan (non-reconstruction range),FIG. 5B is an explanatory diagram during the scan (reconstructionrange), and FIG. 5C is an explanatory diagram at the end of the scan(non-reconstruction range). XB indicates the spread of the X-ray beamlike in FIG. 2, and a square SR shown by the dotted lines shows a scanrange.

First, a doctor or laboratory technician makes the subject P lie on thebed 12 and enters a scan parameter for an image for medical use to beacquired from the subject P. In step ST401, the system control unit 13obtains the entered scan parameter and executes a scan on the basis ofthe scan parameter.

A plurality of wedges 21 (112) are mounted in the gantry 11. In stepST402, a wedge adapted to the scan parameter, the build of the subjectP, the image acquisition direction, and the like is selected. In FIG.5A, wedges 21 a and 21 b are shown, and the wedge 21 a is selected.

In step ST403, execution of a scan is started on the basis of a scanschedule of the non-reconstruction range and the reconstruction rangecalculated by the scan parameter.

In step ST404, at the time point when the revolving speed of therevolving part 15 of the gantry and the moving speed of the bed 12become constant, the slit of the not-shown slit mechanism is opened toapply an X ray, and the scan is started. As shown in FIG. 5A, in thenon-reconstruction range at the start of the scan, the system controlunit 13 controls the wedge driving unit 117 to move the wedge 21 a to aposition where the X ray is not applied on the outside of the scan rangeSR, thereby suppressing spread of the X-ray beam XB. Consequently, thedose of irradiation of the X ray to the subject P in thenon-reconstruction range is reduced to half.

In step ST405, in the reconstruction range after the non-reconstructionrange at the start of the scan, the system control unit 13 furthercontrols the wedge driving unit 117 to move the wedge 21 a in thedirection of the outline arrow shown in FIG. 5B, thereby controlling thefocal point 23 of the X ray to be positioned in the center of the wedge21 a. At this time point, the X-ray beam XB is not interrupted by theX-ray shield 22.

In step ST406, as shown in FIG. 5C, in the non-reconstruction range atthe end of the scan, the wedge 21 a is further moved by the wedgedriving unit 117 to a position where the X ray is not applied on theoutside of the scan range SR, thereby suppressing spread of the X-raybeam XB. Consequently, the dose of the X ray to the subject P in thenon-reconstruction range is reduced to half. In step ST407, the scan isfinished.

By moving the position of the wedge 21 a during the scan execution,without the slit mechanism in which two slits can open/close singularly,the active collimation operation can be performed.

As described above, in the first embodiment, also in the low-price X-rayCT apparatus having the slit mechanism which performs only thesimultaneous opening operation, if the apparatus has the wedge drivingunit, the active collimation can be performed by using the wedge of theembodiment. Since the active collimation operation is enabled only bychanging firmware or software of the wedge driving unit, it greatlycontributes to lower price and higher performance of the apparatus.

Second Embodiment

FIGS. 6A and 6B show a modification of the wedge. FIG. 6A is aperspective view, and FIG. 6B is a cross sectional view taken along theZ-Z′ axis shown by a dotted line, and seen along line X-X′.

In the first embodiment, the X-ray shield 22 is disposed in the upperpart (on the X-ray tube 111 side) of the wedge 21. In the secondembodiment, an X-ray shield 62 is disposed in a side face of a wedge 61.

In the case of disposing the X-ray shield 62 in the side face of thewedge 61, thickness “t” and height of the X-ray shield 62 are determinedin consideration of the spread of the X-ray beam. Width “w” is set toalmost the same as the width of the wedge 61. In the first embodiment,to prevent the opening shape of the wedge 21 from changing, the X-rayshield 22 is additionally disposed on the X-ray tube 111 side of thewedge 21. Due to this, there is the possibility that mechanicaldimensions of the wedge change. However, in the embodiment, the X-rayshield 62 can be embedded in the side face of the wedge 61, so that thewedge can be designed in the same dimensions as those of the originalwedge. Consequently, there is no mechanical restriction such thatattachment to the conventional X-ray CT apparatus is impossible.

As described above, in the second embodiment, in addition to the effectof the first embodiment, the mechanical restriction can be eliminatedand an effect that the wedge of the embodiment can be attached to anyconventional X-ray CT apparatus is produced.

Third Embodiment

There is a specification type of an X-ray CT apparatus in which, at thetime of driving a wedge, position information is reset once to assurethe position precision of the wedge, and the wedge position is reset tothe original position. In the X-ray CT apparatus having such aspecification, the active collimation operation of the embodiment cannotbe realized in a scan cycle time. Consequently, by adding a novelposition detecting method to the operation of movement between theposition where an X ray is blocked in the non-reconstruction range andthe wedge center position in the reconstruction range, the operation canbe realized.

The wedge driving unit 117 of the X-ray CT apparatus of the embodimentwill be described with reference to FIG. 7.

As shown in FIG. 7, the wedge driving unit 117 has a ladder pattern 71disposed for position detection on the top or side face of the wedge112, a position detecting unit 73 for detecting the position of thewedge 112 from photosensors 72 a and 72 b which read the positionalrelation of the ladder pattern 71 disposed on the wedge 112 inaccordance with a reflection ratio of light, a position control unit 74for controlling the wedge 112 between a position where an X ray isblocked and a wedge center position on the basis of an output of theposition detecting unit 73, and a motor for driving the wedge to apredetermined position in response to a control signal from the positioncontrol unit. As necessary, an illumination light 75 which illuminatesthe ladder pattern 71 is added.

For example, the ladder pattern 71 is a pattern of black and white. Inthe position where the wedge 112 blocks the X ray, the photosensor 72 areceives reflection light (reflects by the white pattern), and thephotosensor 72 b does not receive reflection light (absorbs by the blackpattern). When the wedge 112 is in the wedge center position, thephotosensor 72 a does not receive reflection light, and the photosensor72 b has the pattern of receiving reflection light. The photosensors 72a and 72 b are disposed to face the ladder pattern 71 so that theposition relation can be discriminated. In the case where there is aplurality of wedges, the wedges may have different ladder patterns.

Therefore, the photosensor 72 a constructed as described above will becalled a center position sensor 72 a, and the photosensor 72 b will becalled a block position sensor 72 b.

FIG. 8 shows a time chart of the active collimation operation in theembodiment. It illustrates an output waveform 82 a of the centerposition sensor 72 a, and an output waveform 82 b of the block positionsensor 72 b. Using FIG. 8, control on the wedge 112 since the start of ascan to the end of the scan will be described.

To select a predetermined wedge from the plurality of wedges 112, in anarrow indicated by step ST801, the position control unit 74 controls themotor to reset the wedge to the origin position. In step ST802, theposition control unit 74 performs the position control to the centerposition of the selected wedge 112.

In the non-reconstruction range at the scan start, like the arrow shownby step ST803, the motor is reversely rotated to perform the positioncontrol to block position 1, and a scan is started. In the blockposition 1, the output waveform 82 b of the block position sensor 72 bbecomes “ON”, and the output waveform 82 a of the center position sensor72 a becomes “OFF”. In such a manner, the control can be performed.

In the reconstruction range, like the arrow shown by the step ST804, themotor is forwardly rotated to perform control to the center position. Inthe center position, the output waveform 82 a of the center positionsensor 72 a becomes “ON” from “OFF”, and the output waveform 82 b of theblock position sensor 72 b becomes “OFF” from “ON”. In such a manner,the control can be performed.

Further, in the non-reconstruction range close to the scan end, like thearrow shown by the step ST805, the motor is forwardly rotated to performcontrol to the block position 2. In the block position 2, the outputwaveform 82 b of the block position sensor 72 b becomes “ON” from “OFF”,and the output waveform 82 a of the center position sensor 72 a becomes“OFF” from “ON”. In such a manner, the control can be performed.

As described above, in the third embodiment, also in the X-ray CTapparatus having the specification of resetting the wedge position tothe origin position at the time of driving the wedge, the activecollimation operation can be performed. Moreover, the attached ladderpattern 71, the photosensors 72, and the like are inexpensive and theyare small and do not take up much space, so that there is hardlymechanical restriction. Consequently, an effect that the wedge of theembodiment can be attached to any conventional X-ray CT apparatus isproduced.

In the case where there is room in the X-ray CT apparatus, an encoder orthe like may be attached in place of the ladder pattern 71 and thephotosensors 72, and the motor may be controlled by an output of theencoder to realize the active collimation operation.

The present invention is not limited to the foregoing embodiments andcan be variously modified. For example, the shape of the X-ray shieldadded to the wedge shown in the embodiments is merely an example, andvarious shapes can be employed.

Although the X-ray CT apparatus having the revolving part is especiallydescribed in the foregoing embodiments, for example, also in an X-rayapparatus having no revolving part, an X-ray beam can be blockedpartially or entirely by moving a wedge to which an X-ray shield isadded.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. An X-ray CT apparatus comprising: an X-ray source; a wedge which isdisposed between the X-ray source and a subject and in which a shieldblocking a part of an X ray is formed; a wedge driving unit for movingposition of the wedge; and a system control unit controlling the wedgedriving unit during a scan execution period to control the position ofthe wedge.
 2. The X-ray CT apparatus according to claim 1, wherein thesystem control unit controls the wedge to a first position where an Xray, which spreads in a slice thickness direction of the X-ray source,unnecessary for reconstruction of an image for medical use is blocked ina non-reconstruction range at the start of the scan execution period,and controls the wedge to move to a wedge center position so as not toblock the X ray in a reconstruction range of the image for medical use,and further controls the wedge to move to a second position where an Xray unnecessary for reconstruction of the image for medical use isblocked in a non-reconstruction range at the end of the scan.
 3. TheX-ray CT apparatus according to claim 2, wherein the wedge has theshield on the X-ray source side.
 4. The X-ray CT apparatus according toclaim 3, wherein the shield has a shape which does not exert influenceon a fan angle of the X ray.
 5. The X-ray CT apparatus according toclaim 4, wherein the shield is made of lead.
 6. The X-ray CT apparatusaccording to claim 2, wherein the wedge has the shield in a side face ofthe wedge.
 7. The X-ray CT apparatus according to claim 6, wherein theshield has a shape which does not exert influence on a fan angle of theX ray.
 8. The X-ray CT apparatus according to claim 7, wherein theshield is made of lead.
 9. The X-ray CT apparatus according to claim 2,wherein the wedge driving unit has a position detecting unit fordetecting the first and second positions where the X ray is blocked andthe wedge center position.
 10. The X-ray CT apparatus according to claim9, wherein the position detecting unit comprises a block position sensorfor detecting the first and second positions where the X ray is blocked,and a center position sensor for detecting the wedge center position.11. The X-ray CT apparatus according to claim 10, wherein each of theblock position sensor and the center position sensor is constructed by aphotosensor, and a ladder pattern of different reflection ratios isdisposed in the wedge so that reflection light enters the block positionsensor in the first and second block positions, and reflection lightenters the center position sensor in the wedge center position.